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Environmental Processes of the Ice Age: Land, Oceans, Glaciers (EPILOG) Alan C

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Environmental Processes of the Ice Age: Land, Oceans, Glaciers (EPILOG) Alan C Quaternary Science Reviews 20 (2001) 627}657 Environmental processes of the ice age: land, oceans, glaciers (EPILOG) Alan C. Mix! *, Edouard Bard", Ralph Schneider# !College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503, USA "CEREGE, Aix-en-Provence, France #University of Bremen, Bremen, Germany Abstract Knowledge of the state of the earth at the Last Glacial Maximum (LGM, an interval around 21,000 years ago) is an important benchmark for understanding the sensitivity of global environmental systems to change. Much progress in understanding climates of the LGM has occurred in the &20 years since the end of the CLIMAP project of the 1970s (Climate Long-range Investigation, Mapping and Prediction). Here we review this progress, based on presentations and discussion at a "rst open science meeting of the EPILOG project (Environmental Processes of the Ice age: Land, Oceans, Glaciers) held in Delmenhorst, Germany, May 1999. We outline key controversies and document protocols for EPILOG contributions, so that a new synthesis of the LGM Earth can emerge as an open project of the world's community of scientists. ( 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction looked or poorly represented processes that must be understood and incorporated into the next generation of A long-standing goal of paleoclimatologists is to models. At present, con"dence in model predictions of understand climate changes of the last ice age, the largest future climate change is limited, especially because many manifestation of natural climate change that remains of the models incorporate oceanic processes in a limited relatively well preserved in the geological record. This way. interest is more than just academic curiosity. Current If the geologic record is to provide a useful test of debate about the extent of future climate changes, climate models, we must "rst have con"dence that we whether natural or manmade, demands that we under- know what really happened during the last ice age. How stand the extent, rate, and frequency of natural climate cold was it? Was cooling relative to modern climate changes of the past. uniform over the globe, or do geographic patterns of Assessing the likelihood of future changes requires the change reveal the processes responsible for change? What use of computer models that link components to simulate mechanisms set the sensitivity of Earth's climate system the atmosphere, ocean, land surface, ice, and biota. In to large-scale changes? spite of growing sophistication of such e!orts, models inevitably simplify reality and must be tuned to simulate both the present system and its sensitivity to change. This 2. The CLIMAP reconstruction limitation of models underscores the importance of re- constructing and understanding the state of the Earth The challenge of reconstructing the surface of the during the Last Glacial Maximum (LGM). If models are Earth at the LGM was taken up nearly 30 years ago by able to simulate the full range of dynamic climate cha- the CLIMAP (Climate Long-range Investigation, Map- nges actually observed, then we gain con"dence in their ping, And Prediction) project, which built on a method ability to predict the future. If models fail in their simula- for quantitative estimation of past environments de- tions of the past, we learn from the failure about over- veloped by Imbrie and Kipp (1971). During the 1970s, the CLIMAP group pioneered a "rst global reconstruction of the ice-age climate. Along the way they revolutionized * Corresponding author. Tel.: #1-541-737-5212. the study of global environments in the geologic record E-mail address: [email protected] (A.C. Mix). as they developed strategies for quantitative stratigraphy 0277-3791/01/$- see front matter ( 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 1 4 5 - 1 628 A.C. Mix et al. / Quaternary Science Reviews 20 (2001) 627}657 Fig. 1. Changes in the annual average sea surface temperatures (LGM-modern), with geographic distributions of LGM seasonal and perennial sea ice, and LGM land ice, based on seasonal estimates of CLIMAP (1981). A major conclusion of CLIMAP, surprising at the time, was that cooling associated with ice ages was regional, rather than global. on a global scale, established a preliminary chronology Africa) cooled signi"cantly. Lesser cooling was inferred for global climate change, and advanced methods to near the equator in all oceans. The centers of the sub- estimate quantitatively seasonal temperatures based on tropical gyres and the western Paci"c warm pool yielded distributions of fossil species (e.g., Cline and Hays, 1976). the largest surprise; based on CLIMAP's species-based The CLIMAP project culminated in a series of maps methods temperatures in these areas stayed essentially that depicted the state of the Earth's surface at the LGM the same as (and in some cases even warmed relative to) (CLIMAP, 1981). Although these maps emphasized sea- modern conditions. Seasonal contrast increased near the sonal sea-surface temperatures, they also included the equator (implying stronger seasonal trade or monsoon distribution of ice both on land and in the ocean, as well winds), but decreased at high latitudes (due to perennial as vegetation and albedo (re#ectivity) of the land surface. ice cover on land and in the sea). Overall, CLIMAP A primary result of CLIMAP, surprising at the time, (1981) inferred that Earth cooled surprisingly little during was that the magnitude and even direction of temper- the ice age, except at high latitudes. This concept, often ature change was very di!erent between regions rather referred to as `polar ampli"cationa of climate change, than uniform globally (Fig. 1). The largest inferred chan- implies a feedback mechanism driven by increased al- ges were in the mid-to-high latitudes, especially in the bedo associated with expanded snow and ice cover. North Atlantic region bordered by large ice sheets. In the On land, forests shrunk, deserts expanded, and albedo CLIMAP reconstruction, sea ice advanced in the Green- was adjusted accordingly (Peterson et al., 1979). Debate land and Norwegian Seas and persisted through the within the glaciological community left uncertainty summer. Seasonal meltback of sea ice was also dimin- about the extent and thickness of LGM ice sheets. ished in the Southern Ocean. Upwelling systems asso- CLIMAP proposed two options, both based on equilib- ciated with eastern boundary currents (such as o! NW rium pro"les along presumed glacier #owlines (Hughes A.C. Mix et al. / Quaternary Science Reviews 20 (2001) 627}657 629 et al., 1981). A `maximuma model assumed areas of ice As yet, no large-scale re-analysis of the LGM interval sheets as large as possible based on the greatest extent of exists based on recent estimates of ocean temperature, ice moraines or other evidence (in some cases undated) and sheets, and other environmental properties. Recognizing on an interpretation of ice #owlines that yielded a rela- the need for such a synthesis, 57 scientists met in May tively high Laurentide Ice Sheet dominated by a central 1999 at the Hanse Institute for Advanced Study in Del- dome. Signi"cantly expanded marine ice included an menhorst, Germany, to initiate the EPILOG program. Arctic ice shelf that helped to buttress thick ice on land. Research programs from nine nations (Australia, France, A `minimuma model was based on much more restricted Germany, Netherlands, Norway, Russia, Spain, UK, US) ice limits in many places, particularly around shallow were represented, and the e!ort was sponsored by marine basins, and on #owlines inferred from a di!erent IGBP/PAGES and IMAGES. This "rst public gathering interpretation of the glacial geology that in some cases of EPILOG set as its goals: (1) to assess advances made yielded thinner ice sheets. Hughes et al. (1981) acknow- in the 20 years since CLIMAP, "nding points of consen- ledged that real ice sheets, which respond slowly to cli- sus where possible, (2) to outline strategies that will help mate forcing, may not have achieved full equilibrium to resolve remaining con#icts, (3) to set standards for pro"les during the LGM, but given the data available at international contributions to a new community assess- the time there was no reliable basis for reconstructing ment of the LGM, and (4) to outline a plan of action transient behavior of the global ice sheet system. Indeed, leading to a new synthesis. The meeting emphasized Hughes et al. (1981) stated, `Our most important con- current understanding of sea-surface and continental clusion is that the distribution of late Wisconsin-Weich- temperatures at the LGM. selian ice sheets is not well knowna. This remains an Here we report on the views presented at this "rst important consideration, because climate feedback pro- EPILOG meeting. We focus on the rationale for a new cesses associated with ice depend on the regional details synthesis of LGM environments. We review substantial of ice extent and height (Ramstein and Joussaume, 1995; advances in chronology that help to re"ne research strat- Broccoli, 2000). egy and note key chronology issues that remain unresol- ved. We assess strengths and weaknesses of various geologic proxies for surface temperature and other in- 3. Two decades since CLIMAP formation needed for understanding ice-age climate, and note emerging regional syntheses based on new data. Our Over the past 20 years, several elements of the primary motivation is to highlight key controversies that CLIMAP (1981) reconstruction have been called into need resolution, to outline opportunities for research to question. Ice sheets based on regional sea-level data are "ll gaps in knowledge, and to document protocols for thinner and have about 35% less change relative to EPILOG contributions so that a new synthesis can modern ice sheets than those of the CLIMAP maximum emerge as an open project of the world's community of ice sheet and 10% less than the minimum ice sheet scientists interested in the evolution of past and future reconstruction (Fleming et al., 1998; Peltier, 1994, 1998a, climate.
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